1. Field of the Invention
The present invention relates to an oblique projection type display apparatus and, more particularly, to a rear projection display apparatus designed to attain a reduction in depth of the apparatus by oblique projection.
2. Related Background Art
A conventional apparatus of this type is disclosed in WO 97/01787 or the like. FIG. 6 shows the arrangement of a projection optical system. The conventional technique shown in FIG. 6 is designed to obliquely project an image of an image display panel onto a screen without any trapezoidal distortion by using three to four coaxial aspherical reflecting mirrors and one vertical plane mirror.
When embodiments in the above prior art are traced and examined, it was found that a reception angle (corresponding to FNO) for a light beam emerging from an image display panel is a tradeoff for imaging performance (distortion characteristics, in particular), and it was difficult to obtain satisfactory optical performance.
The mirror data in Table 1 and the ray trace diagram of FIG. 6 show the tracing/examination results on an arrangement including three curved reflecting mirrors and one plane mirror. In this tracing/examination, the projection image maximum distortion was 1.1% and projection image average MTF (1 LP/mm) was 30% at projection FNO=6.0. Note that the image display panel display size in this case was 34 mm diagonal and 16:9 aspect ratio, and the enlarged display size on the screen was 60″ (1524 mm) diagonal.
TABLE 1FNO 6.0Magnification44.4Reflectingr (Radius ofd (DistanceSurface NOCurvature [mm])[mm])K (Cone Constant)Panel Surfaceinfinity 215.651 251.360−281.88 02 434.812 238.00 9.9576373−146.786−238.0−6.882988Plane Mirrorinfinity 295.00 0ScreenInfinityReflectingA (AsphericalB (AsphericalC (AsphericalD (AsphericalSurface NOConstant)Constant)Constant)Constant)10 00 020.171926E−6−0.469882E−100.135238E−13−0.158164E−1730.519197E−8−0.112461E−120.210605E−17−0.172466E−22
Of the data in Table 1, the sign of the surface distance d is reversed every time the propagating direction of a light beam from the image display panel is reversed when it is reflected by the curved reflecting mirror. The signs “+” and “−” of the radius of curvature r respectively indicate concave and convex surfaces.